1. Field of the Invention
The present invention relates to a polyorganosiloxane having at least one dialkoxyorganosiloxy group, the organo radical being a substituted amidoalkyl radical. The novel polydiorganosiloxane is preferably used in an RTV1 silicone rubber mixture.
2. Discussion of the Background
There are known single-component silicone rubber mixtures (RTV1) which can be stored in the absence of water and subsequently vulcanized in the presence of water at room temperature to give elastomers. They generally comprise a polymer, usually a linear siloxane crosslinker which must contain readily hydrolyzable groups, a plasticizer, which is usually a methyl-terminated polydimethylsiloxane, and other additives as desired, such as curing accelerators, pigments, processing aids and fillers. The vulcanization of the mixtures may take place under acid conditions, e.g. in the presence of acetoxysilanes, basic conditions, e.g. using aminosilanes, or neutral conditions, e.g. by means of compounds which have oximo or alkoxy groups. RTV1 systems which crosslink under neutral conditions are needed especially if the substrate is not to be affected by the elimination products produced during the curing of the mixtures, for example in the jointing of concrete or of metallic materials. The terms "RTV" and "RTV1" are used in accordance with common practice in the art, to denote "Room Temperature Vulcanizing" and "Single Component RTV", respectfully.
A polymer having OH groups is normally used for formulating RTV1 materials. This procedure is used, for example, with acetate-, oxime- and amine-crosslinking mixtures. However, this method is not successful in the case of alkoxy-crosslinking materials, since these materials become cured as early as during mixing if OH-end-group polymers are used. To avoid this, polymers with alkoxy end groups are used for alkoxy-crosslinking RTV1 systems. Their preparation is known and generally takes place by reacting a hydroxyl-terminated polydiorganosiloxane of a certain viscosity with an alkoxysilane in the presence of a catalyst. The condensation (elimination of alcohol) which takes place in the above system, forms the desired polymer. An .alpha.,.omega.-dihydroxypolydimethylsiloxane is usually used for this reaction, because of the ready availability in industrial quantities and the high reactivity of such a material (inter alia EP 137 883, EP 304 701 and EP 559 045). The alkoxysilane used is usually methyltrimethoxysilane. There are also known processes in which use is made of compounds such as amino-propyltrimethoxysilane, aminoethylaminopropyltrimethoxysilane, glycidoxypropyltrimethoxysilane and methacryloxypropyl-trimethoxysilane. These compounds, react with the hydroxyl-terminated polydimethylsiloxane even in the absence of a catalyst (DE 44 05 851).
In the processes described, it is important that there is complete reaction of the OH end groups with the alkoxysilane. If OH groups remain in the polymer, then premature crosslinking still occurs when the RTV1 mixture is prepared, and this is precisely what should be avoided by reacting the OH end-group polymer with the alkoxysilane. However, in the process described, only the use of methoxysilanes gives polymers which have no residual OH groups. Because methanol is toxic, however, it was desirable to prepare polymers with other than methoxy end groups, for example polymers carrying ethoxy end groups.
In the hitherto known reactions of hydroxyl-terminated polyorganosiloxanes with ethoxysilanes, the reaction between the OH groups and the ethoxysilanes proceeds either only incompletely or very slowly. Polymers which carry diethoxyhydridosiloxy end groups are an exception. These may be prepared, for example, by reacting hydroxyl-terminated polymers with triethoxysilane (DE 41 13 554). The disadvantage of this process is that the triethoxysilane can be handled only with difficulty, and on storage can decompose into compounds which spontaneously ignite. There is also the possibility that hydrogen may evolve on storage of the polymers or of the finished RTV1 mixtures prepared therefrom.
Another way of obtaining polysiloxanes with ethoxy end groups is by hydrosilylation. For example, according to U.S. Pat. No. 4,772,675, 1-(2-triethoxysilylethyl)-1,1,3,3-tetramethyldisiloxane, which is obtainable from vinyltriethoxysilane and 1,1,3,3-tetramethyldisiloxane, can be reacted with a vinyl-terminated polydimethylsiloxane. U.S. Pat. No. 4,962,174 describes the reaction of an H-terminated polydimethylsiloxane with vinyltriethoxysilane. It is, moreover, possible to react, firstly, a hydroxyl-terminated polydimethylsiloxane with 1,1,3,3-tetramethyldisiloxane in the presence of a platinum compound with elimination of hydrogen, to give an H-terminated polymer, which then likewise reacts with vinyltriethoxysilane. However, these processes always need, as starting material, a compound with a dimethylhydrido group, which, however, are difficult to obtain or have complicated methods of preparation.